T-cell hybridomas and thymocytes undergo programmed cell death (apoptosis) when stimulated with reagents that are normally mitogenic. They also die when stimulated with glucocorticoids such as dexamethasone (Dex). Interestingly, the combination of cellular activation and Dex results in cell survival. The mechanism of this mutual antagonism may be at the level of nuclear transcription factors. A transdominant jun (which prevents expression of a functional AP-1) has been introduced into T-cell hybridomas and completely prevents TCR-mediated activation of AP-1. Stable transfectants and transgenic mice expressing this construct are being made to determine if, as anticipated, AP-1 plays an essential role in programmed cell death and/or lymphokine production. Exploration of a physiological role for glucocorticoid-cellular activation antagonism led us to examine the thymus for endogenous glucocorticoid production. Several approaches have revealed that the thymic stroma produces steroids. Furthermore, blocking steroidogenesis in fetal organ culture profoundly affects T-cell maturation, an effect that is largely overcome by the addition of corticosterone to the cultures. Together, these data suggest a heretofore unappreciated role for the glucocorticoids in T-cell development. Transgenic mice in which glucocorticoid receptor antisense will be directed to the thymus are being made. Finally, CD45- T cells that are grossly defective in several aspects of TCR-mediated signaling (including Ca2+ increases) have been transfected with a construct containing only the catalytic domains of this phosphatase. Despite achieving only ~10% of normal levels of expression, the Ca2+ response reverted to normal. Thus, the transmembrane and extracellular portions of this molecule are not essential for it to exhibit a critical functional role in T-cell activation.